Coupon monitor for cathodic protection system

Abstract

An automated coupon monitor for detecting the electrical parameters associated with a pipe, a coupon and a reference electrode utilized with a cathodic protection system. A voltage detector circuit has its output applied to a pair of sample and hold circuits, one for recording E OFF and the other for recording E ON . Series connected between the coupon and the pipe is a zero resistance current detector circuit and an interrupter switch for measuring the pipe-coupon current and periodically interrupting that current to permit measurement, sampling and storing of E OFF . The sample and hold circuits and the interrupter switch are controlled by a microprocessor controller. The circuit provides DC level outputs which can be easily read by a digital multimeter or stored in a data logger for subsequent reading or transmission to another location.

What is claimed is: 
     
       1. A circuit for monitoring electrical parameters associated with a buried metal object, a coupon and a reference electrode in a soil, the monitoring circuit comprising: (a) a voltage detector circuit having an input connected between the reference electrode and the coupon and an output having a magnitude representing the voltage at its input;   (b) a current detector circuit having an input connected between the object and the coupon and an output having a magnitude representing the object-coupon current;   (c) at least one sample and hold circuit connected to at least one of the outputs for sampling and storing at least one of the outputs; and   (d) a control circuit for periodically actuating the sample and hold circuit.   
     
     
       2. A monitoring circuit in accordance with claim 1, wherein a current interrupter switch is interposed in series with the current detector circuit and connected to the control circuit for periodically interrupting the current through the current detecting circuit and wherein the current detector circuit has a substantially zero input resistance. 
     
     
       3. A monitoring circuit in accordance with claim 2 and further comprising second and third sample and hold circuits connected to the control circuit, one sample and hold circuit connected to the output of the current detector circuit for periodically sampling and storing the output of the current detector circuit and the second and third sample and hold circuits connected to the output of the voltage detector circuit for periodically sampling and storing the output of the voltage detector circuit. 
     
     
       4. A monitoring circuit in accordance with claim 1 or claim 2 or claim 3 and further comprising a low pass filter interposed between each detector circuit and the sample and hold circuit connected to its output. 
     
     
       5. A monitoring circuit in accordance with claim 4 wherein the low pass filter is an analog active filter having a pass band below substantially 10 Hz. 
     
     
       6. A monitoring circuit in accordance with claim 4 wherein the object is connected to a node and the coupon is connected to another node and wherein the current detector circuit comprises an op-amp input stage having its inputs connected between the coupon node and the object node, a power amplifier stage driven by the op-amp and having a power supply connected between said power amplifier and a first one of said nodes, and a current detecting resistor at times series connected between an output of the power amplifier and the second one of said nodes for at times maintaining substantially zero volts between the object and the coupon. 
     
     
       7. A monitoring circuit in accordance with claim 4 and further comprising a third sample and hold circuit having an input connected to the output of the voltage detector circuit. 
     
     
       8. A monitoring circuit in accordance with claim 4 wherein each sample and hold circuit has a droop rate on the order of substantially 0.1 millivolt per second. 
     
     
       9. A monitoring circuit in accordance with claim 1 wherein: (a) the current detector circuit has a substantially zero input resistance and comprises an op-amp input stage having its inputs connected between the coupon and the object, a power amplifier stage driven by the op-amp and having a power supply connected between said power amplifier and the object, and a current detecting resistor at times series connected between an output of the power amplifier and the coupon for at times maintaining substantially zero volts between the object and the coupon; and   (b) a current interrupter switch is interposed in series between the resistor and the coupon and connected to the control circuit for periodically interrupting the current through the current detecting circuit, the current interrupter switch comprising at least three switches connected in a pi configuration, a bridging one of the three switches connected between the resistor and the coupon, and each leg of the pi configured switches connected to the input of the op-amp which is not connected to the object.   
     
     
       10. A monitoring circuit in accordance with claim 9 and further comprising: (a) a second sample and hold circuit connected to the control circuit, one sample and hold circuit connected to the output of the current detector circuit for periodically sampling and storing the output of the current detector circuit and the second sample and hold circuit connected to the output of the voltage detector circuit for periodically sampling and storing the output of the voltage detector circuit;   (b) a third sample and hold circuit having an input connected to the output of the voltage detector circuit for periodically sampling and storing the output of the voltage detector circuit; and   (c) a low pass filter interposed between each detector circuit and the sample and hold circuit connected to its output.   
     
     
       11. A monitoring circuit in accordance with claim 10 wherein the low pass filter is an analog active filter having a pass band below substantially 10 Hz and wherein each sample and hold circuit has a droop rate on the order of substantially 0.1 millivolt per second. 
     
     
       12. A method for monitoring electrical parameters associated with a buried object, a coupon and a reference electrode in a soil, the method comprising: (a) detecting, sampling and holding the current, I, through a conductive path connected between the object and the coupon;   (b) opening the circuit through said conductive path to interrupt the object-coupon current; and   (c) sampling and holding the voltage, E OFF , between the coupon and the reference electrode a selected time interval after interrupting the object-coupon current.   
     
     
       13. A method in accordance with claim 12 and further comprising periodically repeating said method steps. 
     
     
       14. A method in accordance with claim 13 and further comprising periodically maintaining substantially zero volts between the object and the coupon and sampling and storing the voltage, E ON , between the coupon and the reference while the coupon-object voltage is substantially zero volts to obtain the voltage between the reference and the object. 
     
     
       15. A method in accordance with claim 12 or claim 13 or claim 14 wherein the object-coupon voltage is maintained at substantially zero by supplying sufficient current from a power amplifier circuit through a current detecting resistor connected in series with the conductive path. 
     
     
       16. A method in accordance with claim 15 and further comprising periodically reading the values of I, E OFF  and E ON  by with a digital multimeter. 
     
     
       17. A method in accordance with claim 15 and further comprising storing a sequence of the stored values of I, E OFF  and E ON  for subsequent transfer and storage. 
     
     
       18. A method in accordance with claim 14 and further comprising periodically reading the values of I, E OFF , and E ON  by with a digital multimeter. 
     
     
       19. A method in accordance with claim 14 and further comprising storing a sequence of the stored values of I, E OFF  and E ON  for subsequent transfer and storage. 
     
     
       20. A method in accordance with claim 14 and further comprising maintaining substantially zero volts between the coupon and object for a time interval which is at least a multiple of ten times the time interval during which the object-coupon current is interrupted.